The present invention relates to an optical pickup device that conducts recording and/or reproducing of information and to an optical element provided on the optical pickup device.
With a tendency toward high density of an optical pickup device, a wavelength of a laser beam is becoming shorter and a numerical aperture of an optical element such as an objective lens is growing greater, in recent years. For example, in the optical pickup device employing a laser beam having a wavelength of 405 nm, a numerical aperture of an objective lens is as great as 0.6 -0.9, and a maximum angle of incidence or emergence of a laser beam for this objective lens is 60-70°, in general.
Incidentally, it is necessary to keep a beam spot in an excellent form, to conduct accurate recording and reproducing for information. Then, to keep a beam spot in an excellent form, it is necessary to make the intensity of light transmitted through an outer peripheral portion where an angle of incidence is large and the reflectance is high to be the same as the intensity of light transmitted through a central portion where the reflectance is low, in the objective lens. With the background of this kind, there are a technology (see Patent Document 1, for example) to make a thickness of an antireflective film to be thicker on the outer circumferential portion than on the central portion, and a technology (see Patent Document 2, for example) to design an antireflective film by designing the wavelength that makes the reflectance to be lowest in the case of entering perpendicularly to be longer than the working wavelength.
(Patent Document 1)
TOKKAIHEI No. 10-160906
(Patent Document 2)
TOKKAI No. 2001-052366
However, when technologies disclosed in the Patent Documents 1 and 2, transmittance for P polarized light and that for S polarized light are dissociated from each other on the outer circumferential portion of the objective lens.
To explain a simple case, it is considered that an optical pickup device includes a glass objective lens which is used for a light flux with wavelength of 405 nm, has numerical aperture of 0.85 (where the maximum incident angle is 65°) and comprises an antireflective film made from a MgF2 monolayer on both sides of the objective lens. The MgF2 monolayer has a thickness of 73 nm and acts as an anti-reflection coating whose reflectance is around 1.4% for light fluxes with wavelength of λ=405 nm and incident angle of 0°.
Herein, the reflectances of P polarized light and S polarized light having wavelength of λ=405 nm and an incident angle of 60°, show 0.6% and 10.6% respectively, that is, a difference of them is about 10%.
As shown in FIG. 13, the optical pickup device which comprises semiconductor laser element 501, polarization beam splitter 502, 1/4  wavelength plate 503, objective lens 504, disc 505, collimating lens 506 and 507 and photodetector 508, is considered in this case. A laser beam emitted by the semiconductor laser element 501 passes though the collimating lens 506 and the polarization beam splitter 502 and becomes a linearly polarized light. Furthermore, after the linearly polarized light passes through the 1/4  wavelength plate 503, it becomes a circularly polarized light and enters into the objective lens 504 . When a light beam with an incident angle of around 60°enters into surface S1 on the objective lens 504, the incident light becomes an elliptically polarized light, because the transmittance of S polarized light is 90%, whereas the transmittance of P polarized light is almost 100% on the surface S1. Then, the elliptically polarized light is emitted from surface S2 on the objective lens 504, having also an incident angle of around 60°. In this case, similarly, the transmittance of S polarized light is 90%, whereas the transmittance of P polarized light is almost 100% on the surface S2. Therefore, because of this transmittance difference between the P polarized light and the S polarized light, the light emitted from the surface S2 is moreover varied from an ideal circularly polarized light. Thereafter, the incident light onto the disc 505 has an opposite polarization rotation because its phase is reversed, in other words, shifted for by π and passes through the surface S2 and the surface S1 in this order. A light that passed through outer circumferential portion of the objective lens 504 becomes the heavily deformed elliptically polarized light after the light beam totally passes four surfaces at last. While, the light that passed around the light axis of the objective lens 504 remains a circularly polarized light with a reversed rotation after it passes through the four surfaces because transmittances of P polarized light and S polarized light are approximately equal nearby this area. So, when the light passed outer circumferential portion and around the optical axis of the objective lens pass through the ¼ wavelength plate 503 and are converted into a linearly polarized light, a displacement is caused between a light vibration plan of each light. Ordinarily, the polarization beam splitter 502 is designed to separate P polarized light and S polarized light which passed around the optical axis of the objective lens. It reduces a quantity of light entering into a photodetector because the displacement in the vibration planes of the light passed through the outer circumferential portion makes loss when the light is separated. Moreover the low transmittance of the S polarized light similarly decreases amount of light to the photodetector. Furthermore, there is a problem that deterioration of amount of light described above makes a precision of information recording and/or reproducing worse.
Further, in an optical pickup device without a ¼ wavelength plate which makes a linearly polarized light enter into an objective lens, transmittance of a light positioned along a radial direction perpendicular to a vibration plane of the linearly polarized light decreases when an incident angle is almost 60°as shown in FIGS. 14(a) and 14(b). Herein, FIG. 14(a) shows a polarized light state before it passes through the objective lens and FIG. 14(b) shows a polarized light state after it passes through the objective lens. When a transmittance along a radial direction parallel to the vibration plane of the linearly polarized light is assumed 1, reduction rate of a transmitted light positioned along a radial direction perpendicular to the vibration plane of the linearly polarized light is 0.65, which is the fourth-power of 0.9. In other words, it arises a difference of 35% in a transmittance according to positions on the objective lens. There are problems that the transmittance difference along the radial direction reduces the amount of incident light to the photodetector and makes a distortion of a focus servo signal, a tracking servo signal and so on, and at last the precision on information recording and/or reproducing turns worse. As shown in the above descriptions, conventional reflection prevention coat is insufficient for lenses having high NA such as an optical pickup lenses for a wavelength λ=405 nm and such lenses require coating films having same transmission characteristics in P polarized light and S polarized light and higher reflection prevention performance.
In a photomagnetic recording apparatus, rotation of a plane of polarization for light needs to be detected, and therefore, it is preferable that transmittance for P polarized light is the same as that for S polarized light in the objective lens.